It's in the StreamSounds folder. It is indeed called al_cantina_band.wav and you can open it like this: http://lucasforums.com/showthread.php?t=210546
As for putting in your own piece and getting it in-game, I'd imagine you'd have to import your song into Audacity or a similar program, make it mono, export it as a RAW file, and then manually change the file extension name to .wav to get a file the game would recognize. And then just drop it in the StreamSounds folder.